The phenomena of aging are deepened as human beings grow older. Particularly, in the case of skin, the change is remarkable. The main phenomena of the skin aging are increase of wrinkles, thickening, inelasticity, dryness, roughness, spot, etc., which are regarded as being attributed to the exposure to the sunlight over a long time. Such phenomena by the sunlight are called photo-aging and are caused by changes of epidermis and dermis due to the sunlight.
It has been reported that photo-aging of skin may be deterred when a cream containing tretinoin (all-trans-retinoic acid), retinol and derivatives thereof, AHAs, etc. are applied to the skin. However, since tretinoin is fat-soluble, it has little absorbability, and also is unstable in the living body, is irritant to the skin, and may cause some side-effects such as skin dryness, wound, scraping, etc. during the latent period. Therefore, there are many problems in using tretinoin as a main component for medicines and cosmetics (see U.S. Pat. No. 4,677,120). On the other hand, Vitamin A, retinol, can hardly be used because it becomes unstable by light, oxygen, heat, lipid-peroxidation, or water. Therefore, in order to use retinol, additional cost should be paid for stabilizing it by anti-oxidants such as BHT, di-α-tocopherol, BHA, ascorbic acid, tocopheryl linolate, etc., or by adopting some means such as liposome or capsule for intercepting from outside effects (see U.S. Pat. No. 6,221,927 and U.S. Pat. No. 5,744,148). For the reasons as explained above, derivatives thereof having improved stability have been developed. However, the derivatives developed until now are still so unstable when exposed to light that they are converted from a crystalline substance to a viscous one in a short time and are decomposed with some color change and formation of peroxides. As a result, some toxic substances that may give undesirable effects may be formed, which makes the use thereof limited (see U.S. Pat. No. 6,183,774). Meanwhile, it is reported that AHAs have superior stability to Vitamin A and facilitate the collagen synthesis. However, mechanisms of such actions are not accurately elucidated, and thus it is restrictively used due to a concern of skin irritation depending on the pH (see U.S. Pat. No. 6,143,309, U.S. Pat. No. 6,022,896 and U.S. Pat. No. 5,962,015).
Therefore, it is keenly required to develop a material having not only the same activity for deterring skin aging as Vitamin A but also non-irritant property and stability (see U.S. Pat. No. 6,180,670 and U.S. Pat. No. 5,863,942).